Whole casein, alpha s-casein, Kappa - casein and Beta - lactoglobulin, all isolated from the same source of cow's milk, were exposed to various chemical agents (i.e. 8M urea, 2-ME, .3M Ca ions, pH 5 and 9 and heat treatment in the presence of glucose and/or galactose) prior to serving as tryptic substrates. The rate of tryptic proteolysis was monitored with a pH-stat. The results indicate that changes in substrates (protein) configuration (treatment - induced) exerts a greater influence on rate of cleavage than does its cleavage - site (arginine and lysine) concentration.